1. Field of the Invention
The present invention relates to a semiconductor package assembly apparatus and method and to a lead forming apparatus and method that removes tin dust from the surface of leads.
2. Description of the Related Art
A typical process for manufacturing a semiconductor package includes: die bonding which attaches a semiconductor chip (a die) to a die pad of a leadframe; wire bonding which connects bonding wires between bonding pads of the semiconductor chip and inner leads of a leadframe; molding which encapsulates the assembled body in an encapsulant, such as an epoxy molding compound; lead plating which performs tin-plating on outer leads of the leadframe to improve the weldability of the outer leads; trimming which cuts dam-bars of the leadframe and removes flash; and lead forming which bends and forms leads to a desired shape.
A conventional lead forming apparatus and method will be described with reference to FIG. 1 and FIG. 2, each of which shows the construction of a conventional lead forming apparatus. Referring to FIG. 1, a lead forming apparatus 100 includes: a supplier part 10 which supplies to apparatus 100 a leadframe 12 having multiple semiconductor packages thereon; a lead forming part 20 which bends the outer leads of semiconductor packages and separates the packages; a collection part 30 which collects the packages; and a package transportation part 40 for transporting the packages from lead forming part 20 to collection part 30. Lead forming part 20 includes a lead cutting die 22, a lead bending die 24, a lead forming die 26, and a tie-bar cutting die 28.
FIG. 2 shows a lead forming apparatus 200 which includes an intermediate post 150. Lead forming apparatus 200 includes: a supplier part 110 which supplies a leadframe 112 having multiple semiconductor packages thereon; a lead forming part 120 which bends the outer leads of the semiconductor packages and separates the packages; a collection part 130 which collects the packages; intermediate post 150 located between lead forming part 120 and collection part 130; and a package transportation part 140 for transporting the packages from lead forming part 120 to collection part 130. Lead forming part 120 includes a lead cutting die 122, a lead bending die 124, a lead forming die 126, and a tie-bar cutting die 128. Package transportation part 140 includes a first package transportation part 142 for transporting packages from lead forming part 120 to intermediate post 150, and a second package transportation part 146 for transporting packages from intermediate post 150 to collection part 130. Intermediate post 150 shortens the longest traveling distance of the packages by providing a temporary stay to the packages. Alternatively, when package transportation part 40 in FIG. 1 travels a long distance from lead forming part 20 to collection part 30, the packages have a greater chance of being dropped and damaged.
FIGS. 3A to 3D illustrate a conventional lead forming process. In FIG. 3A, a leadframe 300 having semiconductor packages 210 thereon is supplied to a lead forming apparatus. Then, a lead cutting die of the lead forming apparatus cuts outer leads 220 as shown in FIG. 3B. Thereafter, a lead bending die bends outer leads 220, and a lead forming die forms outer leads 220 to a desired shape as illustrated in FIG. 3C. Finally, a tie-bar cutting die cuts tie bars 230 connecting packages 210 to leadframe 300 and separates packages 210 from leadframe 300 to leave individualized semiconductor packages 250 as in FIG. 3D.
As mentioned earlier, the outer leads of semiconductor packages are plated with tin (Sn) or lead-tin (Pb--Sn) alloys. Friction between lead forming dies and the outer leads during the lead forming process can partially strip off the tin layer plated on the outer leads and produce "tin dust" that may adhere to the surface of the dies or the leads. As used herein, "tin dust" is not restrict to tin but may be any conductive dust or shavings. Depending on its size, tin dust attached to a lead is commonly called a tin burr or a tin short. FIG. 4A shows a tin burr 260, and FIG. 4B shows a tin short 270. Generally, tin burr 260 is tin dust having a length greater than a cut-off value, and tin short 270 is tin dust having a length greater than one half of lead pitch A.
Since the tin dust may give rise to an electrical short between the outer leads, after completing the lead forming process, all packages are examined with human eyes to find tin dust. Furthermore, a lead forming apparatus is periodically cleaned to remove the tin dust remaining in the apparatus. Both the postforming inspection and the cleaning reduce the productivity of equipment and processes forming semiconductor devices.